1. Field of the Invention
The present invention relates to a production process of a sulfide containing nickel and cobalt, and in more detail, the present invention relates to a production process of a sulfide containing nickel and cobalt by adding a sulfurizing agent under pressurization into an aqueous solution of sulfuric acid containing nickel and cobalt, which is capable of recovering nickel and cobalt as a sulfide in high yield, as well as, enhancing the utilization efficiency of hydrogen sulfide gas.
2. Description of Prior Art
Conventionally, as a process for selectively precipitating and recovering a heavy metal contained in an aqueous solution of sulfuric acid containing impurity elements, a process for precipitating the heavy metal as a sulfide by a sulfurization reaction with adding a sulfurizing agent, has been used widely. For example, there has been proposed a method for controlling the sulfurization of a heavy metal by using a hydrogen sulfide gas as a sulfurizing agent, and by adjusting a concentration of hydrogen sulfide in the vapor phase (for example, refer to Patent Literature 1), or a method for obtaining a sulfide precipitate having good filtering separation property by adding an alkali sulfide as a sulfurizing agent into an acidic aqueous solution containing nickel and cobalt, and adjusting temperature and pH (for example, refer to Patent Literature 2) or the like. However, in any of these proposals, as shown below, there was a problem to be solved in practical view point.
That is, in the method for using a hydrogen sulfide as a sulfurizing agent, because of handling a hydrogen sulfide gas, which is toxic gas, fine reaction control is required in consideration of safety in practical aspect, as well as, because unreacted hydrogen sulfide gas is discharged, an abatement facility thereof is indispensible. In addition, a sulfurization reaction using a hydrogen sulfide generates an acid by a reaction, as shown in the following expression (1), and lowers pH of a reaction solution.MSO4+H2S=MS+H2+SO4  (1)(wherein M represents a heavy metal element, for example, Ni or Co).
Therefore, because re-dissolution of a sulfide occurs when pH becomes equal to or lower than a specific value, depending on an element to be sulfurized, the sulfurization reaction becomes not to proceed. Accordingly, to make the sulfurization reaction proceed efficiently, the sulfurization reaction is performed either by controlling the lowering of pH by adjusting the element concentration of the reaction solution at equal to or lower than specific concentration, or under neutralizing a generating acid with adding an alkali.
On the other hand, in the method for using an alkali sulfide (for example, NaHS, Na2S) as a sulfurizing agent, because the alkali sulfide is one obtained by absorbing and fixing a hydrogen sulfide gas into an aqueous solution of the alkali, and is stable chemically, it can be used simply and conveniently without having a large scale abatement facility. In addition, because the alkali sulfide itself is alkaline in the reaction, it has merit that pH of the reaction solution does not lower and re-dissolution of the sulfide associated therewith does not occur, different from the case of using hydrogen sulfide, and it can be recovered as a metal sulfide in high yield. However, as for a sulfide precipitate generated by using the alkali sulfide, there is a problem that mole ratio S/(Ni+Co) thereof is, for example, from about 1.1 to 1.2, higher value as compared with a sulfide generated by using a hydrogen sulfide, and it is also easily oxidized. That is, in the step for separating and recovering nickel and cobalt, using such a sulfide, there are big problems that a sulfate ion in a solution increases associated with oxidation, as well as, sulfur treatment load increases caused by excess sulfur content. For example, in the step for leaching the above sulfide precipitate using an oxidizing agent such as chlorine, prevention of sulfur dissolution into a solution generated by leaching becomes the most important leaching requisite.
It should be noted that, in recent years, as a hydrometallurgical process of a nickel laterite ore, High Pressure Acid Leach using sulfuric acid has been noticed. This process, different from a pyrometallurgical process, which is a conventional general metallurgical process of a nickel laterite ore, does not include a dry-type step such as a reducing step and a drying step, and composed of wet-type steps consistently, therefore, has merit that is advantageous in view of energy and cost. That is, in the above High Pressure Acid Leach, because nickel and cobalt can be leached selectively for iron, by controlling redox potential and temperature of a leach solution in a pressurized reactor in the leaching step, and fixing an iron, which is a major impurity, onto a leach residue in a form of hematite (Fe2O3), there is a very big merit.
In the High Pressure Acid Leach, there are included, for example, a leaching step for adding a sulfuric acid to slurry of a nickel laterite ore, leaching under high temperature and high pressure and obtaining a leach solution containing nickel and cobalt; a step for separating a solid and liquid of the leach solution; a neutralization step for adjusting pH of the leach solution containing impurity elements together with nickel and cobalt, to form neutralization precipitate slurry containing impurity elements such as iron, and washed mother liquor for nickel recovery; and a sulfurization step for supplying hydrogen sulfide gas into said mother liquor for nickel recovery to form a mixed sulfide of nickel and cobalt and barren liquor (for example, refer to Patent Literature 3).
In the above sulfurization step, it is an important technical problem, in managing the steps economically, to recover, in high yield, a mixed sulfide of nickel and cobalt, generated and obtain barren liquor, in which nickel and cobalt is sufficiently removed, by introducing the above mother liquor for nickel recovery into a pressurized reactor having a pressure resistance, and still more blowing, into the vapor phase of the reactor, gas for sulfurization, which is supplied from a synthesis facility of hydrogen sulfide gas, installed in the above plant, and controlling a sulfurization reaction with hydrogen sulfide gas in the liquid phase. However, as described above, pH lowers with proceeding of the sulfurization reaction, Ni concentration in a reaction completion solution increases at the time of reaction completion, and Ni recovery rate is deteriorated. Therefore, there was a problem that in the case where pH of the starting solution is low, pH of the reaction completion solution becomes low as well, and Ni recovery rate becomes low.
As a measure to solve this, in a method for using the above alkali sulfide as a sulfurizing agent, in order to obtain a sulfide suppressed the oxidation property, there has been disclosed a method for generating a precipitate of the sulfide, by adding the alkali sulfide into the above aqueous solution, after making inside of the reactor to non-oxidative gas atmosphere, while maintaining redox potential (based on an Ag/AgCl electrode) at −300 to 100 mV (for example, refer to Patent Literature 4). Although this method has merits in respect that nickel and cobalt can be recovered as a sulfide in high yield, and respect that enables to respond with a simple and convenient abatement facility and the like, use of the alkali sulfide as a sulfurizing agent has a problem of cost in the case of a solution with relatively low nickel concentration, like the above mother liquor for nickel recovery.
Under such circumstances, it has been desired to be capable of recovering nickel and cobalt as a sulfide in high yield, as well as, enhancing the utilization efficiency of hydrogen sulfide gas, in a production process of a sulfide by adding a hydrogen sulfide gas as a sulfurizing agent under pressurization into an aqueous solution of sulfuric acid containing nickel and cobalt, as a base.
[Patent Literature 1] JP-A-2003-313617 (page 1 and page 2)
[Patent Literature 2] JP-A-6-81050 (page 1 and page 2)
[Patent Literature 3] JP-A-2005-350766 (page 1 and page 2)
[Patent Literature 4] JP-A-2006-144102 (page 1 and page 2)